tests/qtest/migration: Initialize buffer in probe_o_direct_support

tests/qtest/migration: Initialize buffer in probe_o_direct_support

Valgrind complains about the probe_o_direct_support() function reading
from an uninitialized buffer. For probing O_DIRECT support we don't
actually need to write to the file, just make sure the pwrite call
doesn't reject the write. Still, write zeroes to the buffer to
suppress the warning.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20241209204427.17763-3-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>